Call destroyInstance after workers have exited

Router and filter instances cannot be destroyed before all worker
threads have exited. Otherwise there is a risk that data gets deleted
while someone might still be accessing it. Further, since all router
and filter instances are created by the main-thread it is better that
they are deleted by that thread as well (and not by whichever thread
happens to execute service_shutdown()). That will reduce the risk that
some unknown assumptions are violated.

server/core/gateway.cc

@@ -2044,6 +2044,12 @@ int main(int argc, char **argv)
     {
         thread_wait(threads[thread_id]);
     }
+
+    /*<
+     * Destroy the router and filter instances of all services.
+     */
+    service_destroy_instances();
+
     /*<
      * Wait the flush thread.
      */

server/core/maxscale/service.h

@@ -46,11 +46,19 @@ void service_free(SERVICE *service);
 /**
  * @brief Shut all services down
  *
- * Stops all services and calls the destroyInstance entry points for all routers
+ * Turns on the shutdown flag in each service. This should be done as
- * and filter. This should only be called once by the main shutdown code.
+ * part of the MaxScale shutdown.
  */
 void service_shutdown(void);
 
+/**
+ * @brief Destroy all service router and filter instances
+ *
+ * Calls the @c destroyInstance entry point of each service' router and
+ * filters. This should be done after all worker threads have exited.
+ */
+void service_destroy_instances(void);
+
 /**
  * @brief Launch all services
  *

server/core/service.c

@@ -1892,6 +1892,18 @@ void service_shutdown()
     while (svc != NULL)
     {
         svc->svc_do_shutdown = true;
+        svc = svc->next;
+    }
+    spinlock_release(&service_spin);
+}
+
+void service_destroy_instances(void)
+{
+    spinlock_acquire(&service_spin);
+    SERVICE* svc = allServices;
+    while (svc != NULL)
+    {
+        ss_dassert(svc->svc_do_shutdown);
         /* Call destroyInstance hook for routers */
         if (svc->router->destroyInstance && svc->router_instance)
         {